This invention relates to apparatus for measuring and controlling the tension of the material between adjacent mill stands of a continuous rolling mill for producing wires, rods, shaped steel stocks, etc.
According to a prior art method of measuring and controlling the tension of the material between adjacent mill stands, a looper is installed between mill stands or the length of the loop of the material is measured by contactless means, for example a combination of a light source and a photoelectric cell or an electrostatic capacitance measuring device, and the measured tension or loop length is used to control the speed of a mill driving motor or to vary the roll gap of the mill. However, since it is necessary in these methods to bend the material to form the loop, it is impossible to apply them to situations where the thickness of the material is large or the cross-sectional configuration of the material is complicated.
Another method of speed control of the mill driving motor has been proposed wherein the current of the motor of the first mill stand when the material passes therethrough (at this time, the tension of the material is zero) is stored and the speed of the motor for driving the first or second mill stand is controlled so that the current of the motor of the first stand becomes equal to said stored current since the material is subjected to a tension or a compression when it is rolled by the first and second mill stands. In this method, instead of using a current value a rolling torque or a predetermined relationship between the rolling torque and the rolling force can also be used. This method, however is not advantageous because a fixed value of the tension is not used. Moreover, in a continuous rolling mill including three or more stands, it is impossible to judge whether the variation in the current or rolling torque or the relationship between the rolling torque and the rolling force is caused by the variation in the tension before or after a given mill stand, so that it is impossible to judge the polarity of the tension control. Further, it is necessary that before an instant at which the current, rolling torque or the relationship between the rolling torque and the rolling force is stored, the tension of the material between the i th stand and the (i-1)th stand should have been adjusted to a target value, where i is an integer larger than 1. But if the spacings between stands were too small so that the control interval is short, it would become impossible to control the tension of the stands on the downstream side.
Thus, has been no successful method of maintaining the tension of the material between adjacent stands and it has been impossible to control the tension along the entire length of the material being rolled by a continuous rolling mill.